Applications of various fuel cells to automobiles have been studied in recent years along with an increased interest in global warming issues. In the case of a highly efficient solid oxide fuel cell, for example, power is generated by electrochemical reaction of oxygen with hydrogen, carbon monoxide, and other hydrocarbons by using: a gas containing a large amount of hydrogen as a fuel; and oxygen as an oxidant. In some cases, such fuels are obtained by a method of reforming various liquid fuels and supplying reformed gases thus obtained.
Here, one particularity of using a solid oxide fuel cell (SOFC) for an automobile is that the SOFC is frequently started and stopped unlike a stationary cell. Supply of a raw material gas is stopped in the first place in the case of stopping the SOFC. As a consequence, a hydrogen-rich gas remains inside piping extending from a reformer to an anode of the fuel cell as well as an anode exhaust gas pipe.
At this time, if external air enters the cell due to a drop in temperature of the cell which leads to a negative pressure inside the cell, or due to diffusion of the gas during storage, there are risks of rapid combustion attributed to reaction of oxygen contained in the external air with the remaining hydrogen, induction of degradation in performance attributed to oxidation of an electrode catalyst, and the like. Such a fuel cell therefore has a problem of a difficulty in maintaining a stable performance if the fuel cell frequently repeats starting, stopping, and storing operations.
Accordingly, at the time of stopping a fuel cell system, it is necessary to prevent entry of oxygen by supplying an inert gas as typified by nitrogen in order to protect a catalyst located in a hydrogen generator and to protect electrodes of a fuel cell. For example, Japanese Patent Application Publication No. 2005-179081 discloses a method for supplying an inert gas into a fuel cell system by: providing an oxygen remover in a flow passage opened to the atmosphere; and supplying a fuel cell with a gas which is prepared by removing oxygen from the atmosphere.